1. Field of the Invention
The present invention relates to an electric power steering system for the current feedback control of a motor.
2. Description of the Prior Art
The electric power steering system of the prior art carries out current feedback control based on a difference between a current instruction value computed based on the steering state of a steering wheel and the travelling state of a vehicle and a motor drive current value supplied from a battery to a motor for providing assist force to a system for transmitting power from the steering wheel to the wheels, paying attention to the excellent controllability of output torque for the drive current of the motor. That is, the electric power steering system has been designed to ensure quick response for the controllability of the drive current of the motor as the current control characteristics of current feedback control so as to provide a good steering feeling.
In the current feedback control of the prior art, to ensure the response of current feedback control in order to give a good steering feeling to a driver, the gain of current feedback control is enhanced, the control range of a PID controller for current feedback control is widened, or further the response frequency of a current detection sensor is raised. However, these have the following defects.
When the gain of current feedback control is enhanced, there is an upper limit of the gain due to the control range of the PID controller, an internal response delay of a current detection circuit, or the like. When the gain exceeds the upper limit, in the loop transfer characteristics of current feedback control, both a gain margin and a phase margin decrease and the response of current feedback control becomes instable, whereby the motor drive current oscillates shortly. To prevent such inconvenience and optimize control response, it is desired that the phase margin of the loop transfer function of current feedback control should be set to 40 to 60.degree. and the gain margin should be set to 10 to 20 dB. However, when the thus predetermined control characteristics of a controller, that is, the cross frequency of the loop transfer function of current feedback control is almost equal to the resonance frequency of a mechanical system, even if the loop transfer function of current feedback control itself is stable, a current oscillation triggered by mechanical resonance occurs, or a current oscillation is caused by a kind of induction phenomenon that the current oscillation induces torque oscillation or noise. This problem becomes marked when relatively large assist force is required for the motor, that is, when the motor stops while outputting a torque by stopping and holding the steering wheel at a turned position as in the case of turning the steering wheel while the vehicle stops, or when the rotation speed of the motor is extremely low by turning the steering wheel very slowly. Therefore, to prevent oscillation induced by the mechanical resonation frequency and the cross frequency of current feedback control, the cross frequency is made much lower than the resonance frequency of the mechanical system (that is, the gain of current feedback control is made small) to ensure the stability of current control at the sacrifice of the follow-up properties of current control, thereby preventing the oscillation of the output torque of the motor.
As alternative means of stabilizing the drive current control of the motor, it is conceivable to design current feedback control having quick response which can suppress current oscillation at the resonance frequency of the mechanical system without losing the follow-up properties of current control by making the cross frequency of current feedback control much higher than the resonance frequency of the mechanical system. However, to realize this means, the response range of the PID feedback controller must be widened. To this end, an extraordinary high-performance controller is required according to control specifications such as the response of torque control, for example, a high-speed CPU is required when the improvement of the response of hardware such as a current detection circuit and PID control are carried out by software. This is not wise for the construction of a system.